Regarding separation of a mixture, there are various techniques for removing substances (for example, salts) dissolved in a solvent (for example, water), and membrane separation processes have recently been used as a process for energy saving and resource saving. Examples of membranes used in membrane separation processes include, for example, a microfiltration membrane, an ultrafiltration membrane, and a reverse osmosis membrane. In more recent years, membranes that fall between the reverse osmosis membrane and the ultrafiltration membrane (loose RO membrane or NF membrane: nanofiltration membrane) have been developed and used. This technique even allows obtaining drinking water, for example, from sea water, brackish water, and water containing harmful substances, and, in addition, has been used, for example, in the production of industrial ultrapure water, wastewater treatment, and recovery of valuables.
Most of the composite semipermeable membranes that are commercially available at present fall within two types: one having on a porous support membrane a gel layer and an active layer in which polymers are cross-linked; and the other having on a porous support membrane an active layer produced by polycindensation of monomers.
In recent years, sea water desalination using a semipermeable membrane has been attempted and put to practical use in water treatment plants around the world. It is known that a semipermeable membrane generally comprises a porous support membrane coated with a separating functional layer, and that when the separating functional layer is formed from cross-linked aromatic polyamide, the semipermeable membrane has high stiffness due to the presence of benzene rings, an advantage in that the membrane can be readily produced by interfacial polycondensation of aromatic polyfunctional amines with aromatic polyfunctional acid halides, and, in addition, a high salt removal rate and a high permeate flux.
Boron, which is toxic to human bodies, plants, and animals in that, for example, it causes neuropathy and growth inhibition, is contained in large amounts in sea water, and therefore it is important to remove boron in seawater desalination.